In the design of motor vehicles having internal combustion engines, there is the problem that the air throughput through a given main radiator of the vehicle engine must be sufficient in all driving situations. In the process of saving weight, and in particular in the course of demanding an improved level of pedestrian impact protection, it is generally sought to reduce the size of main radiators while maintaining the heat exchanger power. This makes it necessary to optimize the air throughflow through the exchanger face in all operating states if possible. It is known for this purpose to cover a surface of the heat exchanger with a fan cowling, with an electrically driven fan being provided in one region of the fan cowling and flaps which can be opened being provided in another region of the fan cowling. The flaps which can be opened can, depending on the embodiment, be formed as ram pressure flaps or as actively actuable flaps. In the case in which no flaps are provided, the air flow is impeded at relatively high speeds, since the running fan is overblown and there can be no additional air mass flow rate as a result of ram pressure loading. The provision of ram pressure flaps—which are usually arranged horizontally—results at least in a high degree of resistance in partially-open operation, with heated air at least partially flowing back or being recirculated into the suction region of the fan. Such recirculation fundamentally reduces the efficiency of the heat exchanger. There are therefore often limits to the efficiency, both in the case of passive ram pressure flaps and in the case of actively controlled flaps, as a result of intense recirculation effects.